Selection Criteria
Environmental Considerations
In order to select the appropriate enclosure for an application, the environment to which the enclosure will be exposed must be considered. Most installation atmospheres will be at least mildly damaging since air (oxygen) and water can be slightly corrosive.
Upon specifying an enclosure, the following questions need to be answered:
- What corrosive gas, vapor,liquid, or dust particles may be present?
- Is more than one corrosive element present?
- Are these elements unstable at any time?
- Is there exposure to any byproducts of manufacturing processes?
- Is exposure to weather a concern?
- To what temperature will the enclosure will be exposed?
Certain enclosure materials are more corrosion resistant than others. The enclosure material chosen should have corrosion protection properties that can withstand the effects of the application’s corrosive atmosphere.
Painted carbon steel is the most cost effective metallic enclosure material. The paint finish consists of an inner layer of primer and an outer layer of powder coat which produces a durable and scratch resistant surface. In the case of accidental scratches, touch-up paint should be used to cover exposed bare metal. Resistance to solvents, alkalis, and acids is very limited. Painted carbon steel enclosures are mainly used in indoor applications.
Type 304 stainless steel is an iron-based alloy containing between 18% and 20% chromium. Type 304 is oxidation resistant and provides protection from corrosive solvents, alkalis, and some acids. Stainless steel is ideal for applications that are exposed to wash down cleaning processes. Type 304 stainless steel enclosures are used in both indoor and outdoor applications.
Type 316 stainless steel contains molybdenum, which enables it to provide better overall corrosion resistant properties than Type 304. Additional material properties include an improved resistance against sulfates, bromides, chlorine, sea water, high temperatures, and some special acids. Resistance to sea water makes 316 stainless steel ideal for marine environments. Since Type 316 has very good resistance to chemical attack, it is often specified in pharmaceutical manufacturing applications in which excessive metallic contamination must be avoided. Type 316 stainless steel enclosures are used in both indoor and outdoor applications.
Fiberglass reinforced polyester (known as “fiberglass”) is very impact and corrosion resistant. Fiberglass is a lightweight, extremely strong material which consists of plastic reinforced by fine fibers of glass. Fiberglass enclosures are well suited for corrosive, high temperature environments. Some fiberglass enclosures are immersion proof: they can be submerged in water for specific periods of time while fully protecting the contents of the enclosure. The disadvantages of specifying a fiberglass enclosure include color fade and fiber bloom due to extended exposure to sunlight.
A polycarbonate is a very durable plastic formed from a thermoplastic polymer. Polycarbonate has greater impact resistance than fiberglass and suffers no ultra violet light deterioration. Polycarbonate is much easier to modify as compared to fiberglass, producing cleaner holes and cutouts. Polycarbonate enclosures have a longer product life and are more weather resistant as compared to fiberglass enclosures. Polycarbonate enclosures can be specified with a clear door which makes it possible to monitor installed components while keeping the enclosure door closed. Polycarbonate enclosures are typically small in size which makes them ideal for use as instrument and junction boxes for sensitive equipment. Other applications include temperature control monitoring, geophysical sensing, relay modules, keypads, volume controls, and lighting controls.
Along with the enclosure material, it is important to consider the corrosion resistance properties of the remaining components that comprise the enclosure, which oftentimes may be the weakest link in the whole configuration. It is imperative to specify the correct windows, door latches, gaskets, cooling systems, locking accessories, and other options to ensure that the complete enclosure assembly meets the application’s corrosion resistance requirements.
Potential Effects of Electromagnetic Interference
Electromagnetic interference (EMI) is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. EMI resistant properties should also be considered when specifying an enclosure material.
Appropriate questions to ask are:
- Will EMI affect the performance of installed components?
- Will the enclosure be exposed to radio or EMI waves?
Metallic enclosures offer a natural barrier to radio and electromagnetic interference. The EMI protection properties of metallic enclosures can be enhanced by the use of special gaskets. Non-metallic enclosures do not offer EMI protection, but can be upgraded with metallic coatings which are usually applied to the inside of the enclosure.
Applications Determine Strength Requirements
The required material strength needs to be determined when specifying an enclosure.
Appropriate questions to ask are:
- What is the total weight of the components to be housed?
- Does the enclosure need to be tamper resistant?
- Is the enclosure location secured or accessible by many?
- Will both operators and maintenance personnel have access?
In many industrial environments, enclosures must be able to take abuse and continue to maintain a specific level of environmental protection. Metallic enclosures are stronger and able to carry greater loads than non-metallic enclosures, but are susceptible to denting with impact. Non-metallic enclosures are resistant to some impact, but cannot withstand as much weight or support as heavy a load as metallic enclosures.
Weight Considerations
The enclosure material weight may be important to consider during enclosure selection. Pole, wall,and ceiling mount enclosures may need to be hand carried into position.Enclosures made from fiberglass or polycarbonate may be better suited for these installations as they are lighter in weight than metallic enclosures. Some outdoor applications may require a heavier carbon or stainless steel enclosure to protect against weather and tampering issues.
Potential Modifications Impact Selection
Due to material hardness, adding cutouts and holes to stainless steel enclosures is far more difficult than modifying carbon steel enclosures. Modifying painted carbon steel enclosures does require paint touch-up to protect from corrosion in areas where paint was chipped. Fiberglass and polycarbonate are much easier than steel to modify but these materials can splinter and produce a very fine, potentially harmful dust. Modifications to polycarbonate are more precise and cleanup is easier and safer.
Climate Control Requirements
It is important to note that metallic enclosures have the ability to conduct and dissipate heat. Carbon or stainless steel is the best choice of material if thermal transfer is desired. These materials will dissipate heat from inside the enclosure when the ambient temperature is lower than the desired internal temperature.
Fan forced air can also help the heat removal process. If the ambient temperature is higher than the desired internal temperature, then air conditioning (or air to water heat exchangers) may be required to remove heat from inside metallic enclosures.
Fiberglass and polycarbonate enclosures do not conduct heat; therefore any heat transfer would require fans or air conditioning. The material in a non-metallic enclosure essentially acts as an insulator, keeping internal temperature warm when ambient temperature is cool and vice versa.
Conductivity = Metallic vs. Non-metallic
Metallic enclosures conduct electricity and therefore require grounding to ensure operator safety when installing or maintaining internal equipment. Non-metallic enclosures do not conduct electricity; therefore the installed equipment is isolated from the enclosure, providing a safety barrier to operators. With both metallic and non-metallic enclosures, external grounding of the installed equipment is required to ensure a clear path to earth ground.
Covering the Bottom Line
Cost Considerations
The overall enclosure price is very dependent on the cost of the base material. Regardless of cost considerations, the correct material must be chosen based on the application. Any cost savings realized at the beginning of a project will be insignificant compared to long term costs needed to correct problems caused by choosing the wrong enclosure. The correct enclosure must be chosen based on environmental conditions, required strength, modification capabilities, and weight considerations. Strictly based on enclosure cost, 316 stainless steel would be most expensive, followed by 304 stainless steel. Fiberglass and polycarbonate materials would be the next most expensive options, followed by painted carbon steel which would be the least expensive.Pricing may vary based on quantity purchased.
Pricing may also be affected by the manufacturer’s sales volume and the availability of raw materials in their supply chain.
The following two tables are provided as a general reference guide to help select the proper material for a variety of applications. It is also important to consider the temperature range of viewing windows, latches, and gaskets.
From the outside looking in
Window Materials
Technological advances in material science have expanded available options when selecting a window material. Many different options exist to meet specific design needs and cost requirements. Knowledge of these various materials is critical when making a selection based on the requirements of your design solution.
Acrylic (or Plexiglas) is the most cost efficient window material. Acrylic is flexible, lightweight, and has greater impact resistance than glass. Due to its softness, scratches and blemishes can be removed by buffing. Acrylic has optical clarity, filters ultraviolet light, and cracks instead of shattering. Acrylic does not turn yellow with age and is a better insulator than glass. Acrylic has excellent weather resistance.
Polycarbonate (or Lexan) is more expensive than acrylic, but is becoming more popular as a window material. Polycarbonate has many advantages over acrylic. Polycarbonate is more impact and chip resistant, more flexible, and has greater heat stability than acrylic. Polycarbonate does turn yellow over time and is more likely to scratch than acrylic. Advances in coatings for polycarbonate have improved its UV filtering, light transmission, weathering, and scratch resistance characteristics. Some polycarbonates are able to accept screws and tapped holes. A watertight seal is achievable on polycarbonate windows.
Glass had traditionally been the material of choice for windows, but due to the advances in polycarbonates, glass is now becoming less popular. The disadvantages of higher cost, greater weight, inflexibility, and poor insulating properties often outweigh the advantages of visual clarity, heat resistance, and ease of cleaning.
Conclusion
Whatever the application or environment, choosing the right enclosure material will ensure that the components and equipment housed inside will be sufficiently protected from the environment, electronic interference, and physical abuse for years to come. Choosing the appropriate material will make your end product or application aesthetically appealing, while sufficiently protecting the equipment inside and the operators servicing that equipment.
Precision Automation | Specialists in Industrial Automation 