|David Lancaster |
Enclosures that house electronics and other sensitive equipment are only as good as the seal between the door and cabinet. In the past, engineers designing seals that protect components from fluids and the environment have usually opted for a molded shape or solid O-ring. Now, with the advent of reliable hot vulcanization splicing technology, hollow spliced extrusions have become the most dependable and cost-effective sealing solution.
In some cases, such as on enclosures made of weak or thin material, low-closure-force seals may be necessary. The lightweight hinges and fasteners usually used on these containers can’t handle the large loads needed for a solid elastomer seal. To make matters worse, the containers may be opened and closed many times during their life, increasing the possibility for fatigue failures. The only way to reduce closure forces with solid O-ring seals is to use a lower hardness (durometer) polymer.
Solid seals also suffer from clearance gaps when the seal grooves and mating lids aren’t held to tight tolerances. These gap irregularities can be caused by parallelism runout, varying lid thickness, too few fasteners, and thin or weak enclosure materials. Inconsistent gaps create inconsistent compression forces along the seal perimeter. And if seal compression falls too low or goes to zero, the seal could leak. These problems are difficult to solve with solid cross-section seals. In fact, the only way to produce effective seals with solid cross sections is to tighten the enclosure’s tolerances. Using a hollow hot vulcanized spliced gasket, however, lets designers vary the seal’s wall thickness and the polymer’s hardness for optimum closure forces.
In most applications where there is a groove present, a simple, spliced hollow-O cross-section seal gives designers an effective option when it is uneconomical or impossible to manufacture enclosures with tight clearance-gap tolerances. Hollow seals handle more compression (up to about 50%) without huge closure forces or overfilling the gland. Hollow O-rings can also be designed to stay inside the groove without using adhesives even when the enclosure is overturned or the lid is open. This requires an O-ring outer diameter slightly larger than the groove width. It is often referred to as a “friction fit” and usually cannot be done with a solid O-ring without gland overfill or lowering the seal compression.
Some applications don’t have space for a groove and therefore must be sealed between the flat surfaces of the lid and enclosure. This is called flat-panel sealing and is used frequently on larger enclosures.
Hollow gaskets are especially useful for these applications. Enclosures that use flat-panel sealing are not typically fabricated with tight tolerances and are often made from thin metal or plastic. And large enclosures are more susceptible to leakage due to tolerance-gap runout along the sealing perimeter. Low closure force may also be desirable to decrease the loading on hinges and fasteners.
The challenge in designing flat-panel seals is twofold: securely attach the gasket to the door or the enclosure and prevent misalignment or deflection of the door. At least one section of the gasket should be flat to let it seat well against one a flat surface of the enclosure. Therefore round O-ring seals aren’t usually used in these applications.
When deciding on which cross sectional seal geometry to use, designers must consider how gaskets will be attached. One of the more common methods is to back one side of the flat section of a gasket extrusion with a pressure-sensitive adhesive (PSA). Typical cross sections used with PSA are “D,” “P,” and rectangular shapes. PSAs require multiple-angled spliced corners rather than a single 90° butt splice because PSAs cannot bend around a radius corner. An angled-corner spliced part is often supplied in a four-corner rectangular shape commonly referred to as a “picture frame.”
Fasteners, such as bolts, can also secure gaskets. They pass through the gasket’s cross section and into the enclosure. Cross sections used with fasteners include “P” and “double P” shapes.
One of the most critical steps in designing seals is material selection. For atmospheric sealing applications, where hollow extrusions are often used, the two most often used polymers are silicone and ethylene propylene diene methylene (EPDM). But nearly all polymers can be produced hollow if the application involves sealing a fluid that is incompatible with silicone or EPDM.
Sponge is another material sometimes used when engineers want seals with low closure forces. It’s an alternative to hollow and solid seals, but sponge has some undesirable properties that can lead to seal failure. When sponge-sealed lids are left closed for a period of time and then opened, the sponge has a severe compression set, preventing an effective sealing surface from reforming when the enclosure is shut. And the abrasion resistance of sponge isn’t as high as that of solid elastomers, which cause premature seal erosion and failure if the door is open and closed too often. The poorer permeability of sponge can also leak or soak up the sealed fluid over time.
Silicone rubber, on the other hand, has a variety of properties that make it ideal for atmospheric applications. It resists ozone better than organic elastomers, which lets a silicone gasket last upwards of 15 years in outdoor applications without a significant loss of mechanical properties. The temperature range of silicone is much broader (–150 to 400°F) and its compression set is much smaller than typical organic rubbers.
EPDM is an organic rubber often used in environmental sealing applications. EPDM, sometimes referred to as simply EP, is a saturated synthetic rubber. This means that unlike many other organic rubbers, the polymer isn’t easily degraded by sunlight or ozone. Its temperature range is much narrower than that of silicone (–70 to 250°F).
Hollow gaskets offer many advantages over solid and foam counterparts in enclosure applications, including added design compression, friction fits, low closure force, and superior compression set. All of these qualities combined with competitive costs and wide material offerings make for an overall less-expensive and more-robust enclosure.