When starting a new project, many factors need to be considered, among which the most important is what kind of casing should be used to accommodate the user's components. The shell is the facade of the product and the interface for consumers to interact with it. Users hope to ensure that the quality of the shell can fully meet the needs of the application.

According to the experience of shell manufacturer Hammond Manufacturing, 10 common but very important criteria should be kept in mind when choosing a shell.

1. Size
Ideally, the casing will be selected before designing the PCB. This ensures that the size of the circuit board is just right, and if the housing has mounting columns, it also ensures that the position of the mounting holes is just right. Sometimes the opposite situation may occur, and users must select a casing for their circuit board.

How big is a printed circuit board? Is it installed horizontally or vertically? Many plastic shells have molded slots on the sides for direct vertical installation, or molded mounting bosses on the top and bottom for horizontal installation. The side of extruded aluminum casing usually has full-length slots for horizontal installation of printed circuit boards. In addition, it is necessary to consider whether space should be reserved on the outer surface for indicator lights, switches, connectors, cable entrances, and similar components, and how much space should be reserved? What is the height requirement for components or multiple horizontal circuit boards installed on the circuit board?

Hammond's 1591T series shell image
Figure 1: Hammond 1591T series molded plastic casing with internal card guide rails. (Image source: Hammond Manufacturing)

Hammond's 1455 series shell image
Figure 2: Hammond 1455 series extruded aluminum casing with printed circuit board slots. (Image source: Hammond Manufacturing)

2. Installation environment
Where will the product be used? Indoor, outdoor, or both? When installed outdoors, UV stability is a potential issue for plastic casings, so polycarbonate is chosen by default. Do we need to consider impact and vibration issues for mobile applications such as highways and railways? Is there a risk of water or dust intrusion (see point 5 below)? Is it possible to have pollutants, such as oil or chemicals? Is there a possibility of impact damage? Is it expected that there will be extreme high or low temperatures?

ABS is very suitable for indoor and handheld use. Polycarbonate has excellent temperature resistance, UV stability, and impact resistance, making it an ideal material for outdoor and more industrial applications. Die cast aluminum and other metals have excellent impact resistance and temperature resistance.

Hammond's 1554 series shell image
Figure 3: Polycarbonate shell with transparent cover for Hammond 1554 series. (Image source: Hammond Manufacturing)

3. Materials
For small casings, a choice is usually made between plastic (including ABS and polycarbonate) and metal (including die cast, folded, or extruded aluminum and steel). Polycarbonate is usually specifically used outdoors because it is not as brittle or discolored as ABS due to sunlight exposure.

Aluminum shells have excellent impact resistance and inherent conductivity, but in order to maintain the continuity of the entire shell, painted or anodized surfaces must not cover the mating area. Some extruded shells adopt special designs, such as using multiple heat sinks to increase surface area and improve heat dissipation. Die cast aluminum shell is sturdy and durable; Corrosion resistant, conductive, inherently high electromagnetic attenuation, and easy to process. For applications that require good shock resistance, electromagnetic compatibility, or are expected to face high temperatures, dust, or water, die cast casings are an ideal low-cost choice.

4. EMC (Electromagnetic Compatibility)
The plastic casing has a specific drawback, which is the lack of EMC attenuation capability. If the EMI or RFI emitted by internal electronic components or their sensitivity to external electric fields is a hidden danger, the lack of shielding measures may lead to problems. To solve this problem, various materials can be coated on the inner surface of the plastic shell according to project requirements to provide corresponding attenuation. By coating with materials of different thicknesses, the most cost-effective and technically suitable solution can be provided.

For metal casings, as long as the electrical continuity between the top, bottom, and detachable panels can be ensured, their electromagnetic compatibility is sufficient to meet the needs of most commercial applications. The two halves of the casing will be electrically connected using conductive gaskets.

5. Environmental sealing
Will the casing be affected by environmental risks such as water or dust? The relevant international standard is EN 60529, which specifies the protection level represented by IPXX, where the first digit defines the protection level against solid objects and the second digit defines the protection level against water intrusion.

Generally, enclosures with a protection level of IP54 are suitable for general use. If it is to be installed in an environment where dust and water may exist, the protection level of IP66, 67, or 68 is usually achieved by using gaskets. The highest level is IP69K, which can provide high-pressure steam cleaning protection. In North America, the environmental sealing performance of enclosures is typically defined as NEMA (National Electrical Manufacturers Association) protection level. The NEMA protection level also specifies other product characteristics and tests not covered by the IP level, such as functionality under icing conditions, cable connection holes, etc.