Additional knowledge of enclosure materials results in long-term reliability and cost reduction for controls.
Environmental corrosion combined with damage from impact will affect an enclosure's ability to properly protect the controls and can cause a multitude of problems. Problems could include catastrophic and dangerous system collapses, production downtime, injury to people, increased maintenance costs and losses in customers. This is why it has become critical for specifiers to become more knowledgeable about enclosure materials to prevent these complications.
Even though it should be simple enough to make a decision on the best enclosure material for the application, it can become overwhelming. The reason is that all the available enclosure materials will meet the needs for most applications-up to a point. Nonetheless, one material will be superior when looking for long-term reliability and cost reduction.
Most control housings are typically made up of metals, thermoplastics or composite materials. Following is a brief overview of the three main types of enclosure materials describing how they perform relative to preventing corrosion and impact resistance.
Metals such as gold, platinum and palladium typically do not corrode. These metals are rarely used for control housing because of their high cost. Most other metals will corrode at different rates depending on a variety of factors, such as the type of metal and the environment.
Many specifiers for enclosures select painted carbon steel for outdoor applications based on its low initial cost. Even though painted carbon steel enclosures can be a viable option and survive in many environments, specifiers need to be aware these enclosures can still fail if they are placed in areas where the enclosure is exposed to a more corrosive environment. When the adhesion of paint to the carbon steel becomes compromised, the steel can rust.
This potential for failure of painted carbon steel enclosures causes many specifiers to select stainless steel, instead. Stainless steel enclosures are chosen because they are historically known to be corrosion resistant. Stainless steel is unique because the material contains some 10 1/2 percent chromium. Chromium, when in contact with oxygen, forms a natural barrier or film. This film protects the stainless steel from corrosion, allowing it to survive in harsh environments for many years without incident. Once again, specifiers need to be aware there are cases where the stainless steel film can become compromised.
In Figure 1, a stainless steel enclosure has a large area where the film has peeled away and the steel underneath has begun to rust. This rust spot will continue to grow and could eventually leave the controls inside unprotected.
Impact Strength of Metals
Many specifiers choose metal enclosures based on their reputation for strength.
• Strength measures the resistance of a material to failure.
• Material toughness is its ability to withstand sudden impacts.
• Increasing strength, tensile or compression usually decreases toughness and vice versa.
What specifiers may not know is that even though stainless steels have high strength, they exhibit low toughness-allowing them to easily dent. When the stainless steel enclosure becomes dented, the integrity of the box is jeopardized because it will not have a flush seal.
If an enclosure is not flush, the seal loses its capability to be airtight and watertight. In addition, there is the possibility for compromised security as the controls become easy to access.
Please remember, even though stainless steel is strong, its lack of toughness allows it to be easily damaged.
Enclosures With Visible Damage From Impact
The first example (see Figure 2) is a stainless steel enclosure placed on a golf course in Orlando, Fla. The enclosure has been dented by the impact of stray golf balls. This repeated activity will eventually encumber the enclosure door and expose the controls, costing the business more time and money to replace it.
The second example (see Figure 3) is a stainless steel enclosure with visible signs of impact in Westlake, Ohio. It is currently used to house traffic light controls at a busy intersection. The enclosure had been hit with what appeared to be a baseball bat, causing it to open. This presents the owners with the potential problem of exposing young students, walking to school along this route, to the high voltage controls inside-an unimaginable event no owner or engineer would ever want to deal with.
Thermoplastics-such as polycarbonate, polyester, acrylonitrile butadiene styrene (ABS) and polyvinyl chloride (PVC)-offer a degree of corrosion protection beyond painted carbon and stainless steel. The challenge for use of thermoplastics in some applications is that thermoplastics are more susceptible to ultraviolet (UV) exposure and weathering degradation over time. Certain stabilizers are now added to extend the life of the thermoplastic enclosure, but ultimately the nature of the thermoplastics will yield to extended weathering.
Composite fiberglass is not susceptible to rust or other forms of oxidation. Composite fiberglass enclosures also offer excellent chemical resistance to halogens (chlorine and fluorine). Similar to thermoplastics, composite fiberglass materials can be susceptible to exposure from the damaging effects of UV rays. Damage from UV exposure on composite materials is often called fiber blooming.
There are now other options for fiberglass materials. There is a company that has found a way to combat the issue of fiber blooming. More than 10 years ago, Stahlin Non-Metallic Enclosures developed SolarGuard, a patented sheet molding compound formulation that provides molded-in UV resistance. In extensive comparison testing, SolarGuard outperformed other available sheet molding compound (SMC) formulations by as much as 60 percent in its ability to retain gloss and color after exposure to concentrated UV light. SolarGuard is also able to reduce the effects of UV degradation such as surface roughening and fiber blooming.
Figure 4 is a Stahlin Non-Metallic Enclosure with SolarGuard molded into the material. It is currently located outside the Abita Brewing plant near New Orleans, La. The enclosure has had many years of constant exposure to harsh UV rays, extreme heat and humidity without any changes to the visual appearance because of the protection from SolarGuard.
Thermoplastic and Composite Impact Resistance
Thermoplastics and thermosets (composites) exhibit average strength and a high level of toughness. This means they can withstand sudden impact and maintain their shape. Non-metallic materials are the better option when the application is located in an environment where impact damage is possible.
Each of these three enclosure materials-metal, thermoplastics and composites-will protect the controls in a variety of applications. It is important to know the environment and the application where the enclosure will be used because there are documented cases where the material selected for the application did not provide sufficient protection for the controls. Using this information, engineers can specify the best material for the project-guaranteeing long-term reliability and reduce overall cost for the controls.