Environmental corrosion combined with the damage from impact will affect an enclosure’s ability to properly protect the electrical controls, which can cause a multitude of problems, notes US-based non- metallic electrical enclosure manufacturer Stahlin.
Problems could include catastrophic and dangerous system collapses, production downtime, injury to persons, increased maintenance costs and losses in customers. This is why it has become critical for suppliers to become more knowledgeable about enclosure materials to prevent these complications, the company highlights.
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 being all enclosure materials available today will meet the needs for most applications up to a point; nonetheless, nonmetallic sheet moulding composite materials 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.
Metals such as gold, platinum and palladium typically do not corrode. These metals are rarely used for control housing owing to their high cost. Most other metals will corrode at different rates depending on a variety of factors, including the type of metal and the environment.
Many suppliers 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, suppliers need to be aware that these enclosures can still fail if they are placed in areas where the enclosure is exposed to a more corrosive envi- ronment. When the adhesion of paint to the carbon steel becomes compromised, the steel can rust, Stahlin explains.
This potential for failure of painted carbon steel enclosures causes many suppliers to select stainless steel instead. Stainless steel enclosures are chosen as they are historically known to be corrosion resistant.
Stahlin notes that stainless steel is unique as the material contains about 10.5% 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, suppliers need to be aware that there are cases where the stainless steel film can become compromised.
Many suppliers 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 suppliers may not know is, even though stainless steels have high strength, it exhibits low toughness, allowing it to dent easily. When the stainless steel enclosure becomes dented, the integrity of the box is jeopardised, as it will not have a flush seal.
If an enclosure is not flush, it creates a situation where the seal loses its capability to be airtight and watertight. In addi- tion, the possibility for compromised security exists, as the controls become easy to access.
Thermoplastics, such as poly- carbonate, polyester, acrylonitrile butadiene styrene and polyvinyl chloride, offer a degree of corrosion protection beyond painted carbon and stainless steel.
The challenge for use of thermoplastics in some applications is that they are more susceptible to ultraviolet (UV) exposure and weathering degradation over time. Certain stabilisers are now added to extend the life of the thermoplastic enclosure, but ultimately the nature of the thermoplastic will yield to extended weathering.
Composite fibreglass is not susceptible to rust or other forms of oxidation, and these enclosures also offer excellent chemical resistance to halogens, such as chlorine and fluorine.
Similar to thermoplastics, composite fibreglass materials can be susceptible to exposure from the damaging effects of UV rays, known as fibre blooming.
Today there are other options for fibreglass materials, such as Stahlin’s nonmetallic enclosures with SolarGuard sheet molding composite (SMC), which combats fibre blooming.
Stahlin developed SolarGuard, which is a patented sheet molding compound formulation that provides superior molded-in UV resistance.
In extensive comparison testing, SolarGuard outperformed other available SMC formulations by as much as 60% in its ability to retain gloss and colour after exposure to concentrated UV light. SolarGuard is also able to reduce the effects of UV degradation such as surface roughening.
Thermoplastics and thermosets – or composites – exhibit average strength and a high level of toughness. This means it can withstand sudden impact and maintain its shape.
Stahlin concludes that each of these three enclosure materials – metal, thermoplastics and composites materials 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 along with the material knowledge presented here, engineers can specify the best material for project-guaranteeing, long-term reliability and reduce overall cost for the controls.