Friday, July 15, 2011

The Case For Non-Metallic Enclosures Non-Metallic VS. Metallic Enclosures

The Case For Non-Metallic Enclosures
Non-Metallic VS. Metallic Enclosures


Since the early 20th century, protection and packaging for electrical devices, power distribution and electronics has fundamentally defaulted to metal material choices, typically an alloy of steel or aluminum. The latter part of last century brought the discovery and development of new non-metallic materials including composites and plastics. Specific ideal enclosure applications began presenting themselves for these new materials. Initially corrosion of stainless steel in certain aggressive environments yielded way to composite fiberglass materials as a solution to rapid enclosure degradation.

As we begin our journey into the 21st century, even more consideration will be given to non-metallic enclosure materials. As continued material development and discovery provide non-metallic materials to solve ever-increasing demands, both aesthetic and performance based, non-metallic materials will become a preferred choice in many more applications.


Consider an NFL all-Pro linebacker moving at full speed on a direct collision course with a 275 pound opposing running back. Few things in sports compare to the violent tackle when the two meet. Most times the protective equipment they wear allows them to walk away from the tackle, line up and do it all over again. In the world of enclosures, many applications can seem just as violent, certainly just as aggressive either from a physical or environmental abuse perspective. For decades, engineers and specifiers have primarily chosen metal as their enclosure material choice because of their concern of use in an abusive environment. But just like in football, the best choice may not always be the popular or the obvious one. Protection of the Professional football players in the violent tackle noted above falls to the plastics imbedded in their helmets, shoulder pads, and other protective areas. Could you imagine your favorite team sporting metal helmets? The football analogy is meant to give you pause and question our industry practices of material choice. Today's paradigm for enclosures is typically the metal choice; carbon and stainless steel as well as aluminum at times. In North America only about 10% of enclosure uses are non-metallic. Compare this to elsewhere in the world where usage soars to nearly 30%. So the question raised is Should more consideration be given to non-metallics in the enclosure selection process? This article helps support the case for greater use of non-metallic materials.

Choosing an enclosure.

Ultimately selection comes down to optimal performance and value. Often trade offs between performance, acquisition cost and operating cost are made in the process to find the ultimate choice in a unique application. Consider six influential factors in the enclosure specification and how a composite material such as fiberglass stacks up:

Environmental characteristics
Physical characteristics
Electrical characteristics
Material and material utility

1) The foremost motivating characteristic influencing the enclosure choice is environmental. The consideration envelops temperature, chemical, moisture and concern for the physical world of the permanent installation. Whether the environment is hostile or passive, an attempt is made to match capabilities of the enclosure and the anticipated ambient environment. An over-specified enclosure will work effectively in a natural environment but there are severe repercussions for using an under specified enclosure in a hostile environment, thus making Environment the over-riding consideration.

2) Physical characteristics follow the environmental decision.

The most notable are:

Corrosion resistance
Flexural Strength
Impact Resistance
Tensile strength
Sunlight (UV) resistance
Heat Transfer
Radiused edges
Water absorption
Type of access provided to enclosure interior
Cabinet design and bending radius limitations due to design obstruction

Strength measures the resistance of a material to failure, given by the applied stress (or load per unit area, tensile and compression). Strength is a measure of materials ability to withstand stretching or compressing of the material under load. On the other hand the toughness of a material is its ability to withstand sudden impacts. Increasing strength, tensile or compression, usually decreases toughness and vice versa. Whereas steels often have high strength, they exhibit low toughness which means they dent easily and are difficult to drill or penetrate. Thermosets and thermoplastics, or composites, exhibit average strength but high toughness meaning they can withstand sudden impacts and maintain their shape. Today's composites have improved dramatically in that they can now be designed for both high strength and toughness via additives and fiber reinforcements thus further closing the gap between these two choices in their mechanical abilities.

Corrosion is another factor when choosing the right enclosure materials. Stainless steels have an inherent ability to resist corrosive agents. They achieve this through their internal molecular composition. Carbon steels and aluminum have no other means to protect themselves except for external coatings. Thermoset composites are inherently corrosion resistant as well. Their polymer bond strength is what determines the materials ability to withstand harsh environments. Thermoplastic materials are also corrosion resistant but not to the degree you will find with thermosets. The cross linking of polymer chains in a thermoset is key to corrosion resistance as compared to single long polymer chains commonly found in thermoplastic materials.

Every application has its unique demands, and elements of this list do not follow a precise order. Indeed, many of the capabilities are considered to be inherent in certain material choices. An errant or over-estimated choice, however, can have many repercussions in the life cycle of a product. It makes good sense to specify a product that qualifies in almost every category, insuring satisfactory results without regard for the type of installation.

The use of fiberglass non-metallic insures the broadest range of chemical resistance, strength, lightweight, ease of mounting, flammability, safety, sound dampening and proper heat transfer. It also offers great flexural strength, impact resistance, sunlight resistance and compression strength in a soft radius edged, cosmetically pleasing product.

3) Electrical Characteristics also play a significant part. Like the physical, there is a concern for the protection offered for the installed components as well as protection of the enclosure itself. An enclosure that breaks down over time can no longer perform the duties for which it was specified.

Therefore, the following characteristics are important:
Electrical conductivity
Service temperature
Thermal conductivity
Arc resistance

As a non-conductive material, fiberglass non-metallic provides a natural safety barrier between installed electrified components. At the same time, the enclosure must be specifically grounded, insuring an established ground path while also establishing extra grounding security.

A similar issue is raised over thermal conductivity. The enclosure is insulated and does not dissipate heat generated in the cabinet. It also does not conduct heat so it works as an insulator from high ambient temperatures. Metal enclosures are faced with a near opposite consideration but both choices require some forethought in the final installation regarding heat dissipation.

Electromagnetic interference (EMI), interference caused by energy emanating from high voltage equipment, and radio frequency interference (RFI), interference caused by radio waves, are two methods that impair a systems ability to function properly electronically. Thermoplastic and thermoset composite enclosures are ideal choices for harsh or aesthetically pleasing environments, but being non-metallic they provide no EMI/RFI shielding. Metallic materials on the other hand are inherently effective in their ability to shield internal components from stray electromagnetic waves or radio frequencies. Composite materials can be modified to provide a degree of protection against this interference by coating the inside with a highly conductive nickel or copper coating or by imparting the conductivity directly into the composite material itself using carbon fibers or other highly conductive metallic flakes.

4) Material utility is a consideration for machining, cutting, sawing, drilling and modifying the material of choice. User preference plays a significant part in this selection and material familiarity overrides practicality in many instances. Some users simply accept metalworking protocol as a necessary evil, pointing to common tool usage and familiarity with grounding as key factors. Overlooked assets of non-metallics include ease and accuracy of machining compared to metal alternatives as well as the benefits of field modification for the final installation.

5) Appearance is often an understated requisite. In many cases, the conception of a common look is preferred when using steel or stainless. It should be noted that manufacturers of OEM products, particularly in the instrumentation field, make the more subtle choice of non-metallics to establish product identity because it more readily introduces soft lines into product cosmetics. It is easier to establish a custom look with the many variations of molded product than formed metals.

6) Price is very typically stated as the principal reason for product choice, yet in this list, it comes in dead last. There are some presumptions associated with the cost of metals, at least in the form of products with lesser ratings. As environmental considerations and product life are discussed, a choice for a more expensive product such as stainless steel is considered and the cost benefits of non-metallics are frequently overlooked. Non-metallic enclosures are currently as cost effective as any choice available, particularly when viewed over the full product life cycle.

Reasons for price competitiveness of non-metallics follow some simple thoughts: a) High strength to weight ratio: non-metallic materials can often exceed the strength of steel per pound. Since every product requires shipping and mounting at some point in its life, there is an inherent benefit in non-metallics- reduced weight.

b) Durability: the effectiveness of non-metallics in demanding environments means low overall maintenance costs

c) Corrosion: A landmark study was conducted by Battelle Memorial Institute for the National Bureau of Standards (NBS) - currently, The National Institute of Science and Technology - updated in 1995. Detail was summarized in a report entitled: "Economic Effects of Metallic Corrosion in the United States". Annual costs of metallic corrosion are estimated to be about 4.2 percent of the Gross National Product, or about $350 billion annually. $139 billion (40 %) of these costs could be avoided through application of existing technologies and best-known practices. While the number reflects all manner of corrosion, a significant portion relates specifically to the enclosure industry and its aftermath

d) Low maintenance: No rust, no painting, and no surface deterioration means long term use without cost maintenance. Long product life also insures the life of the components housed in the enclosure.

e) Dielectric: Non-metallics are non-conductive and RF transparent.

f) Environmentally tough: in addition to protection in aggressive environments, non-metallics are UV resistant, functional in extreme temperatures, and are salt, air and chemical resistant.

Product cost and life cycle are always important decisions to be made for electrical enclosures. Much of that decision though is based on the environment the enclosure will be subject to and the time duration or life expectancy the enclosure will be in use.

Acquisition cost is primarily related to the material of choice. If the chosen material were a thermoplastic-based material such as ABS or PVC, the acquisition cost would be 60% on average below that of higher end materials such as thermoset composites and 300% below that of stainless steel. However as with anything else, you get what you pay for. The performance of these materials is also much less than that of thermosets, carbon steel, and stainless steel. Thermoset materials offer the best value /performance ratio than any other material. Their acquisition cost can range from 7-25% below that of carbon steel up to 250% below that of stainless steel in most size ranges. Performance is comparable to that of stainless steel and exceeds that of carbon steel.

Life cycle or duty cycle of any material is of concern to the end user and manufacturer. There is an obvious financial investment into a product that is expected to provide a return on that investment over a given time period. Typically, thermoplastic and thermoset composites have a life cycle exceeding 25 years under normal operating conditions. This can change depending on environmental factors such as the extent of corrosive environmental exposure and protection from physical abuse. Metallic alternatives are subject to those same environmental factors and would thus have similar life cycle expectancies. In highly corrosive environments thermoset composites and stainless steel would be expected to last longer than other enclosure materials due to their ability to withstand attacks by aggressive chemicals.

Future Thermoplastic and thermoset composite materials have developed greatly over the last thirty years, and will continue to evolve as new demands and challenges arise. Technology of resins and other additives continue to drive the increased use of composites in common everyday products. Enclosures are also taking advantage of this development initiative in that new materials can be easily developed to solve real world problems. Such uses include anti-microbial composites for use in the food and beverage industries, carbon fibers and carbon nanofibers for use in high-end electronics, and aramide fabrics for use in anti-ballistic applications are just a few examples. Also as we progress into the 21st century environmental factors such as recycling are becoming more prevalent. Development of new composite base materials from renewable resources such as corn and soybeans will continue to grow and new uses for recycled materials will continue to be developed.

Summary Market Acceptance and the challenges to use of non-metallic materials will continue to be broken down as we progress into the future. Education of non-metallic materials and their uses will be key to this continued growth. More and more engineering schools are teaching the use of composites and non-metallic materials in their curriculums as not just a niche use but as a material of general choice. As non-metallic composite materials improve, their acceptance for use will also improve. New developments and uses outside of the electrical industry will only help to fuel this growth. Areas where composites and non-metallic growth are most notable are the boating, automotive, and aerospace industries. These industries help drive the acceptance of composite non-metallic materials throughout North America and the world. There are still challenges ahead in the use of non-metallic materials in electrical enclosures, but most of these are merely educational. Such roadblocks are a lack of knowledge on the proper uses of non-metallic materials and a general lack of knowledge in performance abilities and comparisons to metallic alternatives. The perception today is that non-metallics are inferior to metallic materials. This is not necessarily true, yet it is today's paradigm and our challenge. Further education in the industry as a whole on non-metallic materials and their uses is an on-going process.

Remember to ponder the next time you are watching football & metal helmets?
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