Factors To Consider When Selecting Standard Plastic & Aluminium Enclosures
Sales & Marketing Director
OKW Enclosures Ltd.
Design engineers today are faced with a huge choice when searching for a suitable standard enclosure for their electronics equipment.
In the past the selection of standard off-the-shelf enclosures was limited to rather simple rectangular housings, perhaps with slide-in aluminium front and rear panels, or even a battery compartment. Over the years however, leading manufacturers such as OKW have developed far more advanced case systems.
Today's standard electronic enclosures feature ergonomically contoured external surfaces which are attractive, tactile and support comfortable operation of the controls. The enclosures also have integrated design features which allow easier installation of modern electronic components such as connectors, displays, membrane and mechanical keypads, power supplies and battery cells. The enclosures can also be configured for a far wider range of applications such as handheld, wall mounted or desktop equipment.
When faced with such a huge selection of standard plastic and metal enclosures it is useful to consider the following key points:
First and important point in my list, try to specify the enclosure before you finalise the PCB layout. Being able to modify the PCB to suit the enclosure has many benefits:
- You can specify the smallest possible enclosure
- You can take advantage of the design features of a very wide range of enclosures on the market
- The design of the enclosure may influence the layout of the electronics.
- Being free to select the right looking enclosure will enhance the visual and tactile quality of your device.
- Interconnections and controls on the PCB can be located easily in relation to the external surfaces of the chosen enclosure.
- You can source the most cost effective solution.
Selecting your enclosure after the PCB layout has been finalised has the following disadvantages:
- You will need to specify a larger enclosure in order to ensure your PCB will fit, perhaps even up to 30% bigger than would have been necessary if the enclosure had been specified before the PCB layout was fixed.
- Your choice of suitable enclosures will be reduced significantly.
- Costs of integrating the electronics within the enclosure will be increased, for example extra-machining etc.
How will the electronics be used? Will they be handheld, placed on a desk, wall mounted, fitted in a rack or onto a DIN rail?
Most manufacturers design their electronic enclosures to meet one or several of these applications. There may be different versions or additional accessories to format the enclosures to suit the application, for example mounting kits for fitting the unit on a wall.
Different physical applications may require additional features. For example handheld enclosures that are used outdoor may need to be sealed against the environment, so a rating of IP65 or IP66 will be necessary. Desktop electronics often need to have an inclined control panel for easy operation of the controls and displays, and may also require a carry handle for portable use.
Also, a wall mounted enclosure that is used outdoors will not only require sealing, but must also be made in a material more resistant to UV light, especially in direct sunlight. So a polycarbonate case will be more suitable for outdoor applications than an ABS case.
What material should I choose?
Selecting your enclosure by material type is an important element in the design of your device. For smaller electronics, plastic or aluminium are the two main options. Steel is used mainly for 19" rack and larger electronic and electrical cabinets.
Selecting a plastic enclosure has the following advantages:
- Modern and tactile design
- Light weight
- High quality finish - resistant to scratches
- Ready moulded-in features - battery compartments, recessed areas for keypads, LCD windows, screw bosses for PCBs etc.
- Easy assembly due to fewer components
Disadvantages of plastic enclosures:
- Fixed external dimensions cannot be changed without re-tooling.
- No inherent EMC protection. Internal coatings required
Selecting aluminium enclosures has the following advantages:
- Strength and relative light weight
- Die cast aluminium components provide a modern look to the housing
- Fabricated aluminium enclosures can be easily modified and can be supplied in different external sizes
- Die cast aluminium enclosures can provide excellent EMC performance and sealing of up to IP67, IP68 and IP69K.
Disadvantages of aluminium enclosures over plastic enclosures:
Interfaces and Controls
- Higher cost
- Extra weight
- Fabricated designs have a more complex assembly technique
- Integrated features such as battery compartments, screw bossed etc. not normally provided.
- Paint chips during use
Nearly all electronic devices require a display, controls and interconnections, even if it is as simple as an LED to indicate operation, an on/off switch and a power connection. It is very important therefore to consider the integration of these components within the selected enclosure.
Most new plastic enclosures on the market will include a recessed area for fitting a membrane keypad, as this has become the most common form of controls for smaller electronic devices. If you need a membrane keypad, avoid enclosures without a recessed area as machining large recessed areas is a costly machining operation.
Designers should also consider the required interconnections when selecting the enclosure. Many plastic enclosures have the split line mid way between the top and the base part. This is not always ideal when fitting connectors. Some more advanced enclosures will have the split line off centre to allow a large flat area for mounting connectors on. Some enclosures even include removable end panels for easier drilling and assembly of the connectors.
In the past many enclosures were introduced with a pre-moulded display window for fitting a standard display module. These are still offered, however, today with such a huge and varied selection of display modules available, most enclosure manufacturers will offer to machine a window into the enclosure to suit the exact profile of the module being used.
Plastic enclosures offer the best solution for smaller battery powered devices. Typically the battery compartments provided in these enclosures will accept: 1.5V AAA, 1.5V AA and 9V cells, smaller N cells and round cells are also accommodated on key fob remote control style enclosures.
There are a few important issues concerning battery compartments that need to be considered. Not all manufacturers provide a fully functioning compartment. Many simply provide a rectangular compartment with a clip-on plate. The customer is then expected to use a battery holder inside the compartment. This is not a satisfactory solution.
Modern and advanced enclosures will provide a fully functioning battery compartment which will accept push-in plated steel spring contacts. The contacts should conform to DIN EN 60601 to avoid short circuits should the batteries be wrongly inserted. To avoid this the polarity of the cells should also be moulded into the compartment floor, which is also a legal requirement of battery powered devices.
Some popular Handheld enclosures such as the OKW Datec-Control cases also offer screw-in cradles which will allow different types and numbers of cells to be fitted into the same size of enclosure, for example 2 x AA or 4 x AA depending upon the demand of the electronics.
It is also important to note that having a clip-on lid on a battery compartments will not be acceptable for all applications. For medical applications for example the battery compartment lid must have tooled (screwdriver) access only. Modern enclosures will include the possibility of fitting the lid by fixing screws.
For low voltage cable powered devices it is now possible to specify enclosures which incorporate flexible cable grommets and glands fully integrated into the end of the enclosure without the need for additional machining.
For mains powered devices double insulated plastic or metal enclosures provide the best solutions.
It is true to say that plastic enclosures are generally easier to assemble than their metal counterparts. This is because the moulding process reduces the number of components. There is also an increased use of clip-together components that do not require fixing screws. Clip-together enclosures can offer very fast assembly operations and cleverly designed cases will also include a tool access feature, which requires a flat screw driver or key to open the unit. This will allow the unit to conform to ant-tampering regulations.
Another very important issue concerning the assembly of plastic enclosures is the type of fixing screws supplied. Some enclosures are still supplied with traditional self-tapping screws. These are suitable for only for a very few assembly and disassembly operations. The risk of cross-threading is high, thus rendering the enclosure unusable.
Modern enclosures should be supplied with special plastic self-cutting screws with a course thread that ensures frequent opening operations and reduces the risk of cross-threading
Machine screws and brass inserts are no longer used in plastic enclosures because of the impact they have on recycling. They are still used in the most part for metal enclosures.
For wall mount enclosures and terminal enclosures, captured fixing screws are an important feature for units that are frequently opened for calibration and servicing.
Learn more about enclosures at www.okw.co.uk